After Dramatic Drying, A Pulse of Life for the Big Bend Rio Grande
When the Rio Grande ran dry in Big Bend National Park last spring, photographs of the park's iconic canyons framing an empty riverbed sparked concern not only in Texas, but nationally. The response was justified.
The Rio Grande is many things – a lifeline for communities and farmers, an international border, a cultural touchstone. In Big Bend, it's also a protected wild and scenic river, part of a national park and a UNESCO international biosphere reserve. The glories of its landscape and its life are something in which we all have a stake.
And recently, there have been hopeful developments. Only four months after it ran dry, the Big Bend Rio Grande surged this summer with some of its highest sustained flows in decades. One good year can't reverse the effects of unsustainable human use, or of a warming and drying climate. But it was almost certainly a boon for river health. And more importantly, there's potential for improving the river's long-term prospects. Amidst the complex binational politics of the Rio Grande, questions of the river's ecology here are getting a new hearing.
U.S. Geological Survey scientist David Dean has studied the Big Bend Rio Grande for years.
“This release – it peaks at twice what 2013's release was,” Dean said, “so I'm anticipating when I actually work up the sediment data, there will also have been substantial movement of sediment through the Big Bend reach during this release.”
Dean was talking about dam releases on the Rio Conchos – the Mexican tributary that joins the Rio Grande at Presidio-Ojinaga – which swelled the river here from late summer into fall.
Local summer rains revived the Rio Grande, which, in April and May, had been reduced to a series of isolated pools through much of the national park. But it was a robust monsoon season in the Rio Conchos headwaters, in the Sierra Madre, that produced the sustained high water. Almost no water reaches the Big Bend from the mainstem of the Rio Grande, taxed as it is for municipal and agricultural uses. The Conchos is the primary source of water here.
The heavy rains in early September filled Conchos reservoirs that had been critically low. To prevent catastrophic flooding, Mexican water managers opened the gates at the Luis Leon Dam, and left them open until September 26th.
The flows that followed were good news for Big Bend river outfitters and boaters, and certainly for native fish and other aquatic life. But ecologically, the most important implications were for the river's “sediment surplus.”
With the construction of dams, and modern upstream demands, flows in the Big Bend have decreased by 95 percent from their historic levels. But perhaps the most significant change has been the end of regular flooding. In the absence of regular floods, silt, clay and sand carried in by flashing tributaries accumulate in the river channel. That accumulation has choked much of the Big Bend Rio Grande, transforming it from a broad and braided stream, into a narrow ditch.
The narrowing channel means that when floods do occur, they can be devastating. And there's a high cost for aquatic life, which relies on the diverse niches of a healthy river.
“The reason why managers are aiming to limit the narrowing,” Dean said, “is because as the river narrows, we lose important aquatic habitats for native fish – side channels and backwaters, these low velocity habitats along the channel margin that juvenile native fish use.”
The recent high flows weren't comparable to the big floods that once occurred here twice a decade. But they likely were sufficient, in magnitude and duration, to move sediment, and check the river's ongoing constriction. In the coming months, Dean will analyze data from monitoring devices to quantify those impacts.
And though the high water reflected the anomaly of a wet year, it also suggests a template for the river's long-term restoration.
Stephen Lantz is Big Bend park hydrologist.
“Dave Dean and the work of USGS and others has really demonstrated that it's possible,” Lantz said, “that there are flows that are attainable within the current allocations that come through the Rio Grande that could be used to fix some of what's broken. Essentially, the main goal for me and the park service is to bring forward in the short term this idea that water can be used to move sediment.”
Even in years of normal rainfall, Lantz said, dam releases could be managed to mimic ancient patterns, and to reduce the Big Bend's “sediment surplus” and open the river back up. He's helping make the case to policy makers for such “environmental flows.”
The binational management of the Rio Grande was set in a 1944 treaty. Mexico is required to deliver a fixed quantity of water to the U.S. via the Conchos, or other tributaries, on a five-year cycle. Mexico came up short as the last cycle ended, in 2019. That led to the creation of binational “working groups,” to explore ways to make water deliveries more “reliable and predictable.” They're charged with crafting a “minute” – an addition to the treaty – by the end of this year.
Lantz is part of that process.
The Rio Grande today is a thoroughly “engineered” river. The river as a living system is subsumed to its role as plumbing – for transporting water to cities and farmers. Big Bend National Park isn't a water user – its interest is in the river itself.
“This is a place where water passes,” Lantz said, “when the deliveries from the Rio Conchos are making their way to Amistad. That's why I think we're uniquely positioned to talk about the shape of those deliveries, and maximizing the benefit of the shape of that delivery, and to put that in to the conversation.”
Under an environmental flows program, deliveries on the Conchos could be managed to create high flows in late summer and early fall. That would move sediment flashing creeks had delivered during summer monsoons. Here, Lantz said, what's good for the river is also good for people – fighting channel narrowing reduces the risk of catastrophic flooding in the borderlands.
Other elements of environmental flows could include a pulse of high water in spring, allowing native fish to reproduce. And a minimum flow in the Big Bend could be established – so that last spring's scenes aren't repeated.
Embedding environmental flows in water-management policy could take years. But Lantz said that there's a new enagement with the idea, and a new dynamic of collaboration among hydrologists and policy makers on both sides of the Great River. The long-term fate of the Big Bend Rio Grande – a singular North American resource – could depend on that dynamic.